Electromagnetic radiation absorbers typically are non-conductive composites of one or more kinds of dissipative particles dispersed through dielectric binder materials. The absorption performance of the composite absorber depends predominantly on the electromagnetic interactions of the individual particles with each other and with the binder. For example, Hatakeyama et al. U.S. Pat. No. 4,538,151 discloses an absorber comprising a mixture of: metal or alloy fibers having high electric conductivity, a length from 0.1 mm (100 microns) to 50 mm and a length to diameter ratio ("aspect ratio") larger than 10; ferrite or a ferromagnetic material; a high molecular weight synthetic resin; and, optionally, carbon black.
The term "whiskers" is often used confusingly for both monocrystalline and polycrystalline fibers. For this invention, relatively long fibers are called acicular ("needle-like") whiskers if monocrystalline in structure, or acicular filaments if polycrystalline.
Thickness, weight, and ease of application of the composite absorber are important practical considerations. Accordingly, absorbing paints have also been developed for certain applications. The paints are typically dispersions of the metal/binder composites. For example, Bond U.S. Pat. No. 4,606,848 teaches a paint comprising stainless steel, carbon, or graphite fibers in polyurethane, alkyd, or epoxy binders. The fibers range in length from 10 micron to 3 cm (30,000 micron) as the diameter ranges from 0.01 micron to 30 micron, thus the aspect ratio is a constant 1000.